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Conventional titania photocatalysts consist of a polymeric network of TiO6 units. The novel titania photocatalysts synthesized by Wachs’ group include isolated TiO4 units, polymeric chains with TiO5 structures, rafts consisting of TiO6 coordination, and TiO6-containing titania NPs measuring 1-10 nm. The group is modifying these photocatalytic active sites to optimize the collection of light and the utilization of the formed electrons as well as their corresponding holes, or positive sites. The goal is to establish the fundamental relationships between structure and reactivity for titania-based photocatalysts.
“We want to see if titania’s structure affects the electron’s lifetime,” says Roberts, who has also traveled to France and the Netherlands to work on the project. “We’re trying to determine what’s happening at the molecular level so we can improve titania’s reactivity and efficiency. Our ultimate goal is to find the optimal catalyst for splitting clean water into H2 and O2.
|A sorbent modeled by Caram and Sircar halves the temperature needed to produce hydrogen from methane.|
“So far, our results show that some isolated TiO4 sites could be superior to the block crystal structure. But although they work well in the lab, we need a lot more catalyst for it to be useful on an industrial scale.”
If successful, Wachs’ group could greatly increase the efficiency of producing H2, says Roberts.
“Photocatalysis runs at very low, even room temperatures. Because light is sufficient to make it go, you are eliminating or greatly reducing your dependence on man-made energy.”
Somphonh Peter Phivilay, another Ph.D. candidate in Wachs’ group, recently spent two months in Japan working with photocatalysis pioneers Kazunari Domen and Masakazu Anpo.
“We tested our catalyst there,” says Phivilay, “and were able to see the relationship between structure and reactivity. We split water and used gas chromatography to measure how many micromoles of H2 we produced. We found isolated surface TiO4 sites to be the most efficient for PWS. The process is not yet cost-effective, but we’re making progress.”
Lowering the heat for H2 production
Steam-methane reforming (SMR), the most common method of producing H2, requires a catalytic reactor heated to almost 900 degrees C, a water-gas shift (WGS) reactor, and a multicolumn, multistep pressure swing adsorption process to purify H2. The process gives off carbon dioxide (CO2) as a by-product.
Shivaji Sircar and Hugo Caram, professors of chemical engineering, have developed and simulated a simpler method of producing fuel-cell-grade H2 at less than 500 degrees C. Their three-year project is funded by DOE.
The new process, a three-step cyclic thermal swing sorption-enhanced reaction, uses potassium carbonate K2CO3-promoted hydrotalcite, a chemisorbent, to selectively remove CO2 from the reaction zone of the reformer.
“Our sorbents permit the operation of the SMR reaction at a much lower temperature – approximately 480 degrees C – while still offering more than 90 percent conversion of methane to hydrogen,” says Caram.
“This enables us to directly produce high-purity hydrogen that can be used for fuel cells or commercial applications. and we can also separate out pure CO2, which can be sequestered or used for petroleum recovery.”
By utilizing the chemisorbent, says Caram, the researchers are able to integrate the SMR and WGS reactors and the hydrogen-purification unit of the conventional process into a single unit. The chemisorbent is periodically regenerated through pressure or thermal swing adsorption. and removal of CO2 from the reaction zone allows the high conversion of methane to hydrogen.
In another project funded by DOE, Caram and Sircar utilized K2CO3-promoted hydrotalcite and a second sorbent, sodium oxide (Na2O) promoted alumina, to produce fuel-cell-grade H2 and pure CO2 from synthesis gas. Syngas, which contains CO, CO2, H2O and H2, is produced when coal is gasified with steam at high temperatures.
Sircar and Caram reported their results last year in the journal Adsorption and in the International Journal of Hydrogen Energy. They also receive support from the Pennsylvania Infrastructure Technology Alliance and Air Products and Chemicals Inc.
Promoting the capture of CO2
Engineers at Lehigh’s Energy Research Center (ERC) develop technologies and systems that increase the efficiency of coal-fired power plants while reducing pollution and facilitating the capture and sequestration of CO2. ERC director Edward K. Levy described some of these efforts in a May 2008 presentation to DOE’s Seventh Annual Conference on Carbon Capture and Sequestration.
One new ERC technology achieves this trio of benefits while also providing a new water source for power plants. The technology, tested successfully at a coal-fired power plant, recovers water vapor from the flue gas with six “staged” heat exchangers whose temperatures are varied so that water vapor and vapors from toxic acids, especially sulfuric acid, condense in separate exchangers. (Water in flue gas condenses at 95 to 130 degrees F, and sulfuric acid at 220 to 310 degrees F.) The recovered water can be treated and used to replace water that evaporates from the cooling tower. Funding for the project is provided by DOE.
Researchers say this technology will be especially useful for power plants in arid regions that lack access to cooling water.
“We were able to recover 50 to 80 percent of the water from the flue gas,” said ERC senior research scientist Harun Bilirgen. “We believe this will make it possible to provide as much as 20 percent of the cooling water needed for a 600-MW power plant boiler burning Powder River Basin coal and 30 percent for the same unit if it’s burning lignite.”
The new design will also improve plant efficiency, reduce mercury emissions 60 to 65 percent and capture sulfuric, hydrochloric and other acids, Bilirgen said. And by cooling flue gas and reducing its acid and water vapor content, the technology could cut the costs of capturing CO2 in back-end amine and ammonia CO2 scrubbers.
“The heat exchanger series will be located along the duct that carries flue gas to the stacks,” said Bilirgen. “The existing temperature of 300 degrees F in this duct will drop to less than 110 degrees F after the exchangers.
“This will be very helpful for post-combustion carbon-capture techniques, which require a flue gas temperature of 110 degrees F or lower for efficient operation.”